We report silicon cross-connect filters using microring resonator coupled multimode-interference (MMI) based waveguide crossings. Our experiments reveal that the MMI-based cross-connect filters impose lower crosstalk at the crossing than the conventional cross-connect filters using plain crossings, while offering a nearly symmetric resonance line shape in the drop-port transmission. As a proof-of-concept for cross-connection applications, we demonstrate on a silicon-on-insulator substrate (i) a 4- channel 1×4 linear-cascaded MMI-based cross-connect filter, and (ii) a 2- channel 2×2 array-cascaded MMI-based cross-connect filter.
©2008 Optical Society of America
Recent breakthroughs in silicon photonics [1–6] constitute the basis of on-chip optical communications for high-performance many-core computation [7, 8]. As suggested by K. Bergman’s group , one way to enable such nanophotonic networks-on-chip in a compact footprint is to employ silicon microresonator-based cross-connect switching elements as building blocks and cascade the microresonators in arrays.
Indeed, microresonator-based filter arrays have been attracting continuous research interests for telecommunications applications [4, 9]. In order to enable large-scale-integrated photonic circuits in a compact footprint, the waveguides could intersect one another in a waveguide grid. Although microresonators coupled with conventional waveguide crossings have long been investigated in compound glass [9–11] and silicon nitride [12, 13], there are to-date only a few reports on microresonator-coupled cross-connect filters in silicon-on-insulator (SOI) technology [14, 15].
Conventional waveguide crossings have the shortcomings of scattering loss and crosstalk (power leaks to the intersecting waveguide) at the crossing junction due to wavefront expansion. The scattering loss and crosstalk are particularly severe in high-index-contrast material systems such as SOI. For potential networks-on-chip applications, a typical ~1-dB scattering loss and ~-20-dB crosstalk per crossing  are detrimental. To this end, a low-loss low-crosstalk waveguide crossing is desirable. Over the past decade, researchers have proposed several schemes to improve the waveguide crossing performances [16–21]. Most recently, W. Bogaerts et al. proposed a double etch scheme to fabricate elliptical waveguide crossings, and the reported loss per crossing was ~0.16 dB with crosstalk below -40 dB .
Previously , our research group reported a multimode-interference (MMI) based waveguide crossing in SOI. The principle is based on self-imaging the waveguide mode field from the MMI waveguide input-plane to its center and output-plane. The self-imaging at the MMI-based waveguide crossing (MMI crossing) center counteracts the wavefront expansion, and thus mitigates the scattering loss and crosstalk at the crossing. Moreover, our MMI crossing design features the merits of (i) relatively simple fabrication processes with high tolerance, (ii) a broadband response typical of MMI-based devices, and (iii) a small footprint (our previously reported MMI waveguide width is 1.1 µm and the length is 4.3 µm). We experimentally demonstrated that our optimized silicon MMI crossings exhibited an insertion loss of 0.4 dB, which can be improved upon better fabrication. This represents a 0.9-dB improvement in insertion loss from the control plain waveguide crossing (plain crossing). Our near-field imaging of the crossing throughput- and cross-port transmissions also suggested that the MMI crossing exhibited significantly lower crosstalk than the control plain crossing.
Here, we report the demonstration of silicon cross-connect filters using MMI crossings laterally coupled with microring resonators. We numerically simulate the cross-connect filter and the MMI crossing by means of beam propagation method (BPM) and two-dimensional (2-D) finite-difference time-domain (FDTD) method. We experimentally demonstrate the cross-connect filters on SOI substrates using silicon microelectronics fabrications. We find that the MMI cross-connect filters display nearly symmetric resonance line shapes in the drop-port transmission spectra, suggesting lower crosstalk than the conventional cross-connect filters using plain crossings. As a proof-of-concept for multiple-channel cross-connection applications, we demonstrate a 4-channel 1×4 linear-cascaded MMI cross-connect filter. We also demonstrate a 2-channel 2×2 array-cascaded MMI cross-connect filter.
2. Design and numerical simulations
Figure 1(a) shows the schematic of the silicon cross-connect filter. The filter comprises a MMI crossing, a side-coupled microring resonator, and four waveguide tapers for in/out-coupling with the bus waveguides. The MMI crossing design is based on our previous work . An on-resonance wavelength is crossed to the drop-port via the microring resonator. An off-resonance wavelength is transmitted to the throughput-port via the MMI crossing. Figures 1(b)–(c) show the 2-D FDTD-simulated TE-polarized (electric field parallel to chip) mode-field patterns of the silicon cross-connect filter at an on-resonance wavelength and an off-resonance wavelength. In both cases the crosstalk due to light scattering at the crossing region is insignificant. The cross-connect filter parameters are as follows: Wm=1.1 µm, Lm=4.3 µm, Wt=0.8 µm, Lt=3 µm, Wb=0.3 µm, g=0.2 µm, and R=6.5 µm.
Here, we further detail the MMI crossing design using numerical simulations. Figure 2(a) shows the 2-D FDTD simulated time-averaged TE-polarized field intensity distribution of the silicon MMI crossing at 1550-nm wavelength. The insets depict the schematic cross-sections of the MMI waveguide and the bus waveguide. The waveguides adopt an air-clad rib structure, with a 0.25-µm-thick silicon waveguide core and a 0.05-µm-thick silicon slab layer sitting on a 1-µm-thick buried-oxide (BOX) layer on a silicon substrate. The simulation parameters follow the fabricated devices. In order to account for the vertical dimension in the 2-D FDTD simulations, we use an effective refractive index of 2.947 for the TE-polarized mode in the silicon waveguide core, and an effective refractive index of 1.623 for the TE-polarized mode in the silicon slab layer.
We choose the MMI waveguide width Wm=1.1 µm in order to support only the three lowest-order TE modes. The calculated MMI waveguide length Lm=4.48 µm, following Lm≈2nW 2 m/λ , where λ=1550 nm is the free-space wavelength, and n=2.87 is the effective refractive index for the TE-polarized fundamental mode of the MMI waveguide (according to BPM). The Lm is approximately twice the beat length of the two lowest-order even TE-polarized modes, yielding single self-images of the input-coupling mode at the MMI waveguide center and output-coupling plane (Fig. 2(a)). In order to adiabatically couple the bus waveguide (width Wb=0.4 µm) fundamental mode to the MMI waveguide two lowest-order even modes, we use waveguide tapers of width Wt=0.8 µm and length Lt=3 µm.
Figure 2(b) shows the FDTD-simulated transmission spectra of the MMI crossing with Lm=4.4 µm (red line) and the control plain crossing (blue line) with the same Wb=0.4 µm. The MMI crossing improves the insertion loss by ~ 0.68 dB from that of the plain crossing (from ~-0.8 dB loss per plain crossing to ~-0.12 dB per MMI crossing) over the 1500–1600 nm wavelengths. Figure 2(c) shows that the MMI crossing also mitigates the crosstalk by ~18 dB to ~40 dB, and the back-reflection by ~30 dB.
We note that the use of the waveguide tapers is crucial for optimizing the MMI crossing design. Figure 2(d) shows the FDTD-simulated transmission intensity as a function of Lm at 1550 nm wavelength. With the waveguide tapers, the insertion loss is optimized around Lm=4.45 µm with a relatively large tolerance in Lm. Without the waveguide tapers, the optimized transmission intensity drops to ~-0.3 dB and the optimized Lm shifts to ~4.7 µm with a less tolerance in Lm.
Table 1 summarizes the FDTD-simulated MMI crossing insertion loss, crosstalk, and back-reflection at ~1550 nm wavelengths using four different Wm values (between 1.0 µm and 1.3 µm) and the corresponding optimized Lm values . Each Wm only supports the three lowest-order TE-polarized modes. The design with Wm=1.1 µm and Lm=4.4 µm gives the optimized -0.12-dB insertion loss, below -40-dB crosstalk, and ~-45-dB back-reflection.
We design and fabricate the cross-connect filters using MMI crossings. We employ standard silicon complementary metal-oxide-semiconductor (CMOS) microelectronics fabrication processes to fabricate the devices on a commercially available SOI wafer (340-nm-thick silicon layer on 1-µm-thick BOX layer). We first thin the device layer down to ~250 nm by wet-etching, and pattern onto the thinned SOI wafer using photolithography (i-line, 365 nm) followed by reactive ion etching. The resulting silicon wire waveguide sits on a slab layer of ~50 nm thick. The slab layer is necessary for dopant implantations in active silicon microresonator-based electro-optic switches .
Figure 3(a) shows the scanning electron micrograph (SEM) of our typical fabricated silicon cross-connect filter using a single microring resonator-coupled MMI crossing. The inset shows the zoom-in view SEM of the MMI crossing. The fabricated MMI crossings follow the same design parameters as those in Ref. , and as shown in our above simulations, except that we only adopt Lm values of 4.3 µm, 4.5 µm, and 4.7 µm. The fabricated bus waveguide width is ~0.4 µm. The microring resonator has a designed round-cornered square shape with corner radius r≈25 µm, and four straight interaction lengths Lc≈15 µm for enhancing the side-coupling between the microring and the bus waveguides. The coupling gap is resolution-limited to ~0.35 µm by the photolithography.
We characterize the cross-connect filter transmissions by end-firing the laser light (1530–1580 nm wavelengths) into the bus waveguide end-face (with a tapered lateral width of ~2.5 µm) using a lensed polarization-maintaining single-mode fiber. The spectral resolution is ~0.02 nm. The throughput- and drop-port transmission light intensities are normalized to the lensed fiber output intensity.
Figures 3(b) and 3(c) show the measured TE-polarized throughput- and drop-port transmission spectra of a single cross-connect filter using MMI crossing (Lm=4.3 µm) as shown in Fig. 3(a), and a control single cross-connect filter using plain crossing. The control filter has the identical bus waveguides and microring resonator as the MMI filter. In both cases, the resonances Q-factors are ~104. The MMI crossing filter exhibits nearly symmetric resonance line shapes in the drop-port transmission spectrum (green line in Fig. 3(b)). Whereas, the plain crossing filter displays pronounced asymmetric resonance line shapes in the drop-port transmission spectrum (pink line in Fig. 3(c)).
In our measurements, the MMI cross-connect filters consistently display lower crosstalks, and more symmetric drop-port resonances than the plain cross-connect filters. The reason for the asymmetric resonance line shape in the cross-connect filter drop-port transmission is fundamental [12, 24]. For a waveguide crossing laterally coupled microresonator-based filter, the drop-port transmission resonance exhibits asymmetric line shape due to the interference between the Lorentzian resonance field coupled through the microresonator and the coherent background field scattered from the crossing junction. This is a classical analogue to Fano resonance , commonly observed in optical resonance systems with coherent background interference. Our MMI crossing enables symmetric resonance line shape in the drop-port transmission spectrum due to the suppressed crosstalk.
We note that the drop-port peak transmissions in Figs. 3(b) and 3(c) exhibit only low transmission efficiency, while the MMI crossing filter shows even lower drop efficiency than the plain crossing filter. We mention that only with proper matching among the waveguide-to-microring input- and output-coupling efficiencies and the resonator round-trip loss can we attain respectable drop efficiency. For our experimental devices (both MMI and plain), we did not optimize the coupling or the loss. Besides, due to fabrication imperfections, the waveguide propagation loss is considerable (~20 dB/cm), which renders a high intrinsic resonator loss. The non-uniform coupling efficiency of the waveguide input- and output-coupling to the microring (due to varied line widths and gap spacings) also leads to drop efficiency variation.
As a proof-of-concept for multiple-channel cross-connection applications, we fabricate a 4-channel 1×4 linear-cascaded MMI-based cross-connect filter. Figure 4(a) shows the SEM of our fabricated linear-cascaded cross-connect filter. The four MMI crossings are identical with Lm=4.7 µm. The four microring resonators differ slightly in the microring corner radii r1,2,3,4 by a fixed increment of Δr=0.2 µm from r1=20 µm, namely r2=r1+Δr, r3=r1+2Δr, and r4=r1+3Δr. We fix the microring interaction length Lc=15 µm for all the four microring resonators. Figure 4(b) shows the measured TE-polarized throughput-port transmission spectrum for our fabricated 4-channel filter. We discern four resonances, denoted as 1, 2, 3, and 4. The Q values are ~104. Figures 4(c) and 4(d) show the measured TE-polarized drop-port transmission spectra for the four drop-ports D1, D2, D3, and D4. The drop-port resonance peak wavelengths align with the throughput-port resonance dips. We also observe nearly symmetric resonance line shapes in the drop-port transmissions.
It is worth mentioning that the certain baseline in all the measured drop-port transmission spectra (both MMI and plain) is not only from the crossing junction induced crosstalk, but also from the propagation modes guided in the slab layer. We estimate the level of slab modes to be approximately -52 dB based on near-field images of the chip output-facet (not shown here).
We also fabricate a 2-channel 2×2 MMI cross-grid array filter, with three microring resonators (of corner radii r1, r2, and r3) coupled to a two-dimensional cascaded array of four identical MMI waveguide crossings [26–28]. Figure 5(a) shows the SEM of our fabricated 2×2 MMI-based cross-grid filter. We design r1=r2=20 µm in order to investigate second-order filtering response for the drop-port transmission at port D1, whereas r3=20.3 µm to separate the two channels filtered by microring resonators R1 and R3. We fix Lc=18 µm for all the three microring resonators.
Figure 5(b) shows the measured TE-polarized throughput-port transmission spectra at ports T and Tr when light is input-coupled from ports I1 and Ir, respectively. The transmission spectrum at port T reveals two resonances corresponding to microring resonators R1 and R3. Whereas, the transmission spectrum at port Tr reveals single resonances corresponding to microring resonator R2. The microring resonator R2 resonances align with one of the two resonances in the transmission spectrum at port T, which is consistent with the design r2=r1.
Figures 5(c)–(d) show the measured TE-polarized drop-port transmission spectra at ports D1, D2, and D3 when light is input-coupled from port I1. The transmission resonances observed at port D3 corresponds to microring resonator R3. Whereas, the transmission resonances observed at port D2 corresponds to microring resonator R1. We note that the transmission dips at port D2 at the resonance center wavelengths correspond to the resonance light that is out-coupled by microring resonator R2. Consequently, the drop-port transmission spectrum at port D1 yields second-order filtering response given by the cascaded nearly identical microring resonators R1 and R2. The second-order filtering bandwidth in port-D1 transmission is 0.09 nm, while the first-order filtering bandwidth in port-D3 transmission is 0.19 nm.
We remark that such form of passive 2×2 cross-grid filter displaying different order filtering (different bandwidths) for the two channels can be undesirable for networks-on-chip applications. However, our point here is to demonstrate the feasibility of cascading the MMI cross-connect filters in a two-dimensional array. For networks-on-chip interconnection as recently proposed in Ref. , the active N×N cross-grid filter (N=5) with identical reconfigurable microresonators only supports first-order filtering for each optical route. Nonetheless, high-order filtering does happen in continuous optical routes through multiple N×N cross-grid filters in the whole network.
In conclusion, we discussed in detail our study of silicon cross-connect filters using microring resonators coupled multimode-interference (MMI) based waveguide crossings in silicon-on-insulator substrates. In particular, we detailed the low-loss low-crosstalk silicon MMI-based waveguide crossing design using two-dimensional finite-difference time-domain simulations and beam propagation method. Our experiments revealed that the MMI-based crossing filter exhibits more symmetric resonance line shape at the drop-port transmission compared with the conventional plain waveguide crossing filter due to crosstalk mitigation. Furthermore, we demonstrated a 4-channel 1×4 linear-cascaded MMI-based cross-connect filter, and a 2- channel 2×2 array-cascaded MMI-based cross-connect filter. We envision that the microring resonator-coupled MMI-based waveguide cross-connect structure can constitute the building block for nascent photonic networks-on-chip applications, namely our recently proposed 5×5 optical switch .
We gratefully acknowledge many fruitful discussions with Prof. Chi Ying Tsui and Prof. Jiang Xu from HKUST. The research was substantially supported by grants from the Research Grants Council of The Hong Kong Special Administrative Region, China (Project no. HKUST6254/04E and Project no. 618707).
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